The present invention relates to a method for operating a voltage controlled oscillator, particularly in a portable communications appliance, with the oscillator being supplied with a variable control voltage which is taken from an operating voltage, preferably from a constant voltage source.
The present invention also relates to an electrical circuit arrangement for controlling a voltage controlled oscillator, with the oscillator having at least one control input for a variable control voltage, and at least one input for an operating voltage.
The present invention furthermore relates to a phase locked loop having at least one detector which produces a variable control voltage, having a filter arrangement and having an oscillator, with the oscillator having at least one control input for the variable control voltage, and at least one input for an operating voltage.
Voltage controlled oscillators are used in communications technology appliances in order to produce radio-frequency sinusoidal voltages. In these oscillators, the output frequency can be varied or tuned within a specific range via a control input. In this case, the magnitude of this tunable frequency range, which is also referred to as the pull-in range, is defined by minimum and maximum voltage which can be supplied to the control input. In this case, the maximum voltage is governed by the available operating voltage.
Two requirements often conflict with one another during operation of modern communications appliances. These communications appliances frequently require the oscillator to have a wide pull-in range, but at the same time operate with low operating voltages; for example, because they are battery-powered. Oscillators which cover a wide pull-in range with only a low control voltage admittedly can be produced, but they often suffer from high sensitivity and poor phase noise characteristics. This is further exacerbated by the use of such oscillators in a phase locked loop.
Until now, in order to solve this problem, the operating voltage of the overall appliance has, for example, been increased by using DC/DC converters or charge parts. However, this solution is expensive, and there is a risk of spectral impurities due to AF interference (AF=audio frequency). Furthermore, a number of oscillators are used, each of which covers only a portion of the desired frequency range. However, this variant now results in enormous costs. Switched oscillators are used in another proposal, in which a sudden frequency change can be produced by switching the resonator circuit, generally via PIN diodes. However, in this case, the design and configuration of such an oscillator are problematic.
An object of the present invention is, thus, to provide a method which allows the pull-in range of an oscillator to be extended in a simple and disturbance-free manner. A further object of the present invention is to provide an electrical circuit arrangement and a phase locked loop for carrying out the method.
Accordingly, the inventor proposes the further development of a method for operating a voltage controlled oscillator, particularly in a portable communications appliance, with the oscillator being supplied with a variable control voltage, which is taken from an operating voltage, preferably from a constant voltage source, such that the variable control voltage is supplied via a capacitor to the oscillator, and a specific additional voltage is added to the variable control voltage when required in an operating phase, with the specific additional voltage being inclined to the capacitor in a preparation phase. The method for operating the voltage controlled oscillator is thus broken down into a preparation phase and an operating phase.
The capacitor is advantageously charged during the preparation phase with the aid of a charging voltage, which is applied to the capacitor. This charging voltage now may be equal to the operating voltage, so that, at the end of the preparation phase, the capacitor is at a voltage (additional voltage) which is equal to the operating voltage. In the operating phase, this additional voltage may be added to the variable control voltage, as a result of which the input voltage range of the oscillator is increased by the magnitude of the additional voltage, and the pull-in range of the oscillator is consequently widened.
One advantageous embodiment of the method according to the present invention provides for the charging time to be controlled by interrupting the charging voltage which is applied to the capacitor. The charging process of the capacitor in the preparation phase may be interrupted, for example, via a control apparatus, which opens a switch after a predetermined charging time, and thus interrupts the charging of the capacitor. The additional voltage which is now present across the capacitor is less than the charging voltage, or less than the operating voltage. The magnitude of the additional voltage across the capacitor is thus dependent on the duration of its charging time. This further development allows for the control voltage to be continuously variably increased, so that the pull-in range of the oscillator can be tuned over a wide frequency range.
A further embodiment of the method according to the present invention provides for an additional voltage to be produced which is in the opposite sense to the charging voltage. This makes it possible for negative additional voltages to be present across the capacitor and, henceforth, even greater variability to be achieved.
In another embodiment of the method according to the present invention, when a low control voltage is desired across the oscillator rather than a high control voltage, the capacitor is discharged to a voltage 0 during the preparation phase. The control voltage, unchanged by the capacitor, is thus produced across the oscillator in the operating phase.
Furthermore, the capacitor is neither charged nor discharged during the operating phase.
In addition, the method according to the present invention is further developed such that the method is carried out on the control voltage of a phase locked loop. A specific additional voltage thus can also be produced here when required during a preparation phase which, in an operating phase, is added to the variable control voltage of a phase locked loop. This results in the input voltage range of the oscillator of a phase locked loop, and hence also its pull-in range, being increased.
The method according to the present invention is thus substantially suitable for those oscillators or phase locked loops which are not operating continuously.
The inventor furthermore proposes the development of an electrical circuit arrangement for controlling a voltage controlled oscillator, with the oscillator having at least one control input for a variable control voltage, and at least one input for an operating voltage, such that a capacitor is provided in series with the control input of the oscillator, and may be precharged in a preparation phase. If an additional voltage, which is produced during the precharging, is now added to the control voltage, a higher control voltage can be made available to the oscillator at its control input.
In one preferred embodiment of the electrical circuit arrangement according to the present invention, at least one switching device is provided for precharging, with the capacitor being charged with the aid of a charging voltage depending on the position of the switching device. At least three, and possibly four, switching devices are advantageously provided for precharging.
A further advantageous embodiment of the electrical circuit arrangement according to the present invention provides for at least one switching device to represent at least one transistor, at least one diode or at least one switch. The switching device also may be formed by combinations of transistors, diodes and switches.
One advantageous further embodiment of the electrical circuit arrangement according to the present invention is to provide a control apparatus for operating at least one switching device. This control apparatus interrupts the additional voltage of the capacitor by interrupting the charging voltage.
The control apparatus furthermore can be designed such that the charging time can be varied. For example, it may contain a timer which causes the control apparatus to interrupt the charging process after a predetermined charging time. By way of example, the capacitor may be charged via a resistor or a current source before the control apparatus opens a switch, and hence ends the charging process. Furthermore, the control apparatus may be in the form of a program part.
Since most voltage controlled oscillators are not operated in a free-running manner, but are part of a phase locked loop, the inventor proposes the further development of a phase locked loop having at least one detector which produces a variable control voltage, having a filter arrangement and having an oscillator, with the oscillator having at least one control input for the variable control voltage and at least one input for an operating voltage, such that the electrical circuit arrangement according to the present invention as mentioned above is provided upstream of the control input of the oscillator. The filter arrangement is, for example, a loop filter, through which DC voltages can pass.
In one preferred embodiment of the phase locked loop according to the present invention, the detector has two complimentary arranged transistors, which each carry out the function of a switching device in a preparation phase. The transistors which are presents in the detector are thus also used for capacitor charging. As such, the capacitor can be both charged and discharged depending on the position of the transistors and a further switch. After the preparation phase, that is to say in the operating phase, the transistors once again operate as phase detectors.
Further features of the present invention can be found in the following description of a number of exemplary embodiments, with reference to the drawings, with an operating voltage of 2.8 V and a control voltage of 0.3 V to 2.2 V being assumed. It is also assumed that the control input of the oscillator has only a negligible current consumption (good approximation for varactor-controlled oscillators).